The present invention relates to a method for centering an electron beam in vacuum coating installations, where the electron beam is guided in a vacuum chamber from a cathode to a raw material source. A focusing field is produced for directing the electron beam to the surface of the raw material source which is thereby heated and melted down. The raw material source is moved up to follow this melt-down. The invention also includes an apparatus for the execution of the method.
Several methods are known for making possible the vaporization of coating materials and for coating the surfaces of substrates with these materials in a vacuum chamber. It is also known to introduce into the vacuum chamber additional reactive gases, such as nitrogen or oxygen, whereby surface coatings of respective compounds result. In German patent document 28 23 876 such a method and apparatus are described. In this reference a substrate carrier for carrying the parts to be coated is arranged concentrically around the main axis in a vacuum chamber. The parts to be coated may include such tools as milling cutters or drills or other objects that are to be provided with a surface coating that is to have special mechanical, chemical or optical properties. In the center of the installation, an electron beam originating from a cathode arrangement and impinging on a raw material source, is guided along the longitudinal axis of the vacuum chamber. Due to the high energy of the electron beam, the surface of the raw material source is fused and the fused raw material is applied onto the substrate by the method described.
By means of a magnetic field the electron beam is focused and aimed at the center of the raw material source. The substrate carrier and the objects to be coated which are disposed on the substrate carrier, disturb this magnetic field. These disturbances cause the electron beam to be deflected and therefore the raw material source is no longer acted upon in the desired manner. Additional difficulties result if the substrate carrier or the objects to be coated are rotated or otherwise moved during the coating process. In this case the influences on the magnetic field change continuously, and the electron beam is deflected and disturbed unevenly. As a result, the raw material source is acted upon and fused unevenly. Also, the focal spot produced by the electron beam on the source may, in an extreme case, impinge outside the source and onto the surrounding mount, and this may lead to corresponding temperature rises and, if cooling is insufficient, to damage to these parts of the installation.
In the known apparatus a raw material source is used which is disposed in a crucible and can contain only a certain quantity of coating material.
For installations with a high vaporization rate it is known also to use, as the raw material source, a billet or bar which can be moved up continuously through the bottom of the crucible. This raw material bar forms the anode, and with the magnetic field uninfluenced, the focal spot formed by the electron beam impinges on the center of the raw material bar and causes uniform melting of its surface. If the electron beam is deflected from the center, the bar melts down on one side, forming a unilateral pasty collar. This unilateral collar is also cooled less well, as it is farther away from the cooled mount. The pasty part of the collar reacts with the reactive gas present in the vacuum chamber, and the resulting compound usually has a higher melting point than the pure material. This also leads to disturbances in the coating process, as thereby the vaporization rate is changed and the melt-down process disturbed. The consequence of this may be that the coating process must be interrupted and the object to be coated may be of inferior quality or must even be discarded.